In a small skillet over medium heat, toast the coriander seeds, cumin seeds, fennel seeds, and the dried chile until fragrant, 40 seconds to 2 minutes. Dump the toasted spices into a spice grinder and process until finely ground. In a blender or food processor, combine the quartered onion, garlic, ginger, and the halved green chile and purée until smooth. If using a food processor, it may take several minutes for the mixture to become completely smooth.
In a heavy-bottomed saucepan over medium heat, warm the oil. Add the sliced onion and the puréed onion mixture and season well. Cook, stirring occasionally, until beginning to brown, 10 to 12 minutes.
Stir in the ground spices, turmeric, and mustard seeds and cook for 2 minutes.
Add the tomato passata, tamarind purée, stock, and coconut and bring to a boil. Add the cauliflower, reduce heat to low, then simmer until the cauliflower is knife tender, about 25 minutes.
Serve the curry with cilantro leaves, sliced green chile, steamed rice, and mango chutney.